Sternum

The two lateral halves of the sternum may fail to unite and
thereby form a cleft sternum (so-called fissura sterni). In other
cases, the union of the two halves occurs in the manubrial region but
fails distally. The cranial and caudal parts may be fused but the
intermediate part remains separate. The failure to fuse can be very
slight or very extensive and is sometimes accompanied by ectopia
cordis.

A more common variation in the sternum is asymmetry of the costal
cartilages, which may articulate in an alternating pattern.

Cartilaginous nodules or suprasternal ossicles may occur singly or
paired at the superior margin of the manubrium. The frequency of
occurrence is reported to be 6-7% of cadavers studied. By x-ray
examination, Cobb found separate or fused suprasternal bones in 6.8%
of adult whites and in 2.2% of adult Negroes; the highest incidence,
7.6%, was in white females.

The joint between the manubrium and the body of the sternum may
become ossified. The manubrium sometimes extends to the insertion of
the third costal cartilages. This usually occurs in gibbons and
occasionally in other anthropoid apes.

The body of the sternum may be perforated, usually at its lower
end.

The xiphoid process is typically variable in size and shape (it
may be cleft) and may be absent. An abnormally long xiphoid,
extending to a level of the umbilicus, has been reported. The xiphoid
process may be perforated, more frequently than is the body of the
sternum.

The body of the male sternum is relatively longer than that of the
female; in males it is more than twice the length of the manubrium
whereas in females it is usually less than twice the length.

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